In a fuel cell a fuel, which is typically hydrogen or an alcohol, such as methanol or ethanol, is oxidised at a fuel electrode (anode) and oxygen, typically from air, is reduced at an oxygen electrode (cathode) to produce an electric current and form product water. An electrolyte is required which is in contact with both electrodes and which may be alkaline or acidic, liquid or solid.
To assist the oxidation and reduction reactions that take place at the anode and the cathode, catalysts are used. Precious metals, and in particular platinum, have been found to be the most efficient and stable electrocatalyst for fuel cells operating at temperatures below 300° C. The platinum electrocatalyst is typically provided as very small particles (˜2-5 nm) of high surface area, which are often, but not always, distributed on and supported by larger macroscopic electrically conductive particles to provide a desired catalyst loading. Conducting carbons are typically the preferred material to support the catalyst.
One type of fuel cell is known as the proton exchange membrane fuel cell (PEMFC) and uses a solid polymer electrolyte membrane which is capable of conducting protons, typically based on perfluorosulphonic acid (PFSA) polymers, such as Du Pont Nafion®. Since these membranes require hydration in order to function, PEMFCs generally operate at temperatures lower than 120° C. The membrane is sandwiched between an anode and a cathode to form a membrane electrode assembly (MEA). For many applications, PEMFCs utilise hydrogen as the fuel source and oxygen, from air, as the oxidant. Impurities in the hydrogen and pollutants in the air can adversely affect the anode and cathode catalyst layers and, in severe cases, permanently damage the MEA. The impurities in hydrogen (carbon monoxide (CO), carbon dioxide, hydrogen sulphide, ammonia, organic sulphur compounds and carbon-hydrogen compounds), arise mainly from the process by which hydrogen is produced. Air pollutants such as nitrogen oxides, sulphur oxides and additional carbon monoxide, carbon dioxide and hydrocarbons, arise mainly from vehicle exhaust and industrial emissions. It has been found that even trace amounts of impurities present in either the fuel or air streams can severely poison the anode, cathode and membrane—particularly at low temperature operation (i.e. <100° C.). Poisoning of any one of these components can result in a performance drop of the MEA. Significant progress has been made in identifying fuel cell contamination sources and understanding the effect of these contaminants on performance. Three major effects have been identified: (1) kinetic effect (poisoning of the electrode catalysts); (2) conductivity effect (increase in the solid electrolyte resistance, including that of the membrane and catalyst layer ionomer), and (3) mass transfer effects (catalyst layer structure and hydrophobicity changes causing a mass transfer problem).
The hydrogen used as a direct fuel in PEMFC technologies is produced by reformation of hydrocarbons and/or oxygenated hydrocarbons, including methane from natural gases and methanol from biomass, and is the dominant method for hydrogen production, although electrolysis is playing an increasing role. The reforming process of hydrogen production results in unavoidable impurities such as carbon oxides including carbon monoxide and carbon dioxide, along with sulphur compounds including hydrogen sulphide and sulphur organics. Steam reforming and partial oxidation or auto-thermal reforming are usually used to produce hydrogen-rich gases known as “reformate” which may contain 25% carbon dioxide, 1-2% carbon monoxide and sulphur impurities in addition to the 70% hydrogen desired. Since hydrogen fuel contaminants can severely hinder PEMFC performance, intensive research activities into the investigation of anode impurities have been conducted. The most extensively studied contaminants are carbon oxides, particularly carbon monoxide, due to the high proportion of hydrogen used in fuel cells being produced through the reforming process.
The problems generated by carbon monoxide in a fuel cell are very well known in the fuel cell community. It is well documented that carbon monoxide binds strongly to platinum sites, resulting in the reduction of surface active sites available for hydrogen adsorption and oxidation. This catalyst poisoning reduces electro-oxidation rates and raises electrode over-potentials, resulting in reduced MEA performance compared to operation under carbon monoxide-free hydrogen. The extent of performance loss due to carbon monoxide poisoning is strongly related to the concentration of carbon monoxide, the exposure time, cell operation temperature and anode catalyst type.
In the case of low level carbon monoxide exposure, poisoning can be reversible through use of an air bleed on the anode, where a small amount of air (1-6%) is injected into the anode gas stream whereby the anode catalyst oxidises carbon monoxide to carbon dioxide in the presence of hydrogen. Although cell performance can be fully recoverable, this is not always the case. Owing to the resulting reduction in fuel efficiency (since fuel is also consumed in this process) and the possible generation of damaging, localised hot-spots, there is increasing resistance from stack manufacturers to use this system measure to prevent/reduce the poisoning effect of carbon monoxide.
An alternative approach to improve the carbon monoxide tolerance of the electrocatalyst is to increase the cell operating temperature. A factor of 20 increase in anode carbon monoxide tolerance has been reported by elevating the cell operating temperature Tcell>100° C. The capability to run at higher operation temperatures and with higher carbon monoxide levels would enable a reduction in the complexity, size and thus cost of the cooling, balance of plant components. However, high operating temperatures impose a new set of materials challenges, not least increased instability of the platinum and carbon against corrosion, dissolution and sintering mechanisms in addition to the absence of any high-performance membrane that can function effectively without sustained hydration.
A further approach has been the development of an electrocatalyst with improved intrinsic tolerance to carbon monoxide, for example by the design and development of PtRu alloy catalysts. Unfortunately, the use of Ru containing catalysts can also be problematic—particularly over life-time tests. The oxidation of ruthenium causes its dissolution and, owing to water fluxes in the MEA, solubilised ruthenium can crossover to the cathode. Ruthenium leaching from the anode and deposition on the cathode has a dramatic effect on the oxygen reduction reaction (ORR) activity of the cathode catalyst since ruthenium deposits onto platinum and remains stable on its surface in the electrode potential window of the oxygen reduction reaction.
Due to these operational issues proton exchange membrane fuel cells for use in automotive applications are required to operate on nominally pure hydrogen, since even low levels of carbon monoxide are found to cause poisoning and loss of performance. The US Department of Energy (DoE) has recently published an updated Hydrogen Quality specification which is required for fuel cells employed in the automotive sector. According to this revised specification, only very low levels of carbon monoxide (0.2 μm/mol, or 0.2 ppm) will be present in any future fuel stream. This CO contaminant limit is in agreement with the recently finalised ISO/DIS 14687-2, Hydrogen Fuel—Product Specification—Part 2: proton exchange membrane (PEM) fuel cell applications for road vehicles, approved by the International Organization for Standardization (ISO). Even with the continued requirement to reduce the total platinum group metal (PGM) loading of MEAs (which would increase the sensitivity of the electrocatalyst layers to impurities), it was generally thought unlikely that this very low level of carbon monoxide would prove problematic during operation. However, in July 2012 the National Renewable Energy Laboratory (NREL) published the results of a DoE supported study (National Fuel Cell Electric Vehicle Learning Demonstration, Final Report) which assessed US fuel cell vehicle technology from 2005 to September 2011. Consideration of the hydrogen production costs and efficiency, production rates and hydrogen quality formed part of this very comprehensive study. Over the six year period 152,000 kg hydrogen was produced or dispensed—from both natural gas reformation and water electrolysis methodologies. The data produced shows that although the carbon monoxide levels in the hydrogen fuel were clearly low, nevertheless levels of up to 1 ppm were measurable.